The frequency domain interferometry (FDI) technique has been developed for probing thin layered structures of the atmosphere. The position and thickness of a single layer embedded within the scattering volume can be deduced from the complex normalized cross correlation (coherence) of received signals at two closely spaced frequencies. Applied in the vertical pointing direction, this technique identified layered structures (FDI layers) of 50--200 m in thickness in the lower atmosphere. These structures are 1 order of magnitude thicker than observed temperature sheets (about 10-m thick) which are very likely responsible for the main part of the VHF radar echoes in vertical direction. In this paper, although the ambiguity of the dual FDI technique is well known, we emphasize that the FDI layers do not necessarily correspond to a single atmospheric layer; they can also be interpreted as a more complex structure of very thin atmospheric layers. A simple model, introduced as an example, shows that the FDI layer thickness can also approximately be interpreted as the vertical separation of two very thin atmospheric layers. This result can explain by itself the differences between the estimated thicknesses by balloon and FDI radar techniques. Finally, we stress that comparisons with high-resolution in situ measurements are urgently needed for interpreting the FDI layers. ¿ 1999 American Geophysical Union